1. Field of the Invention
The present invention relates to an active matrix display, particularly to a full range active matrix display and a driving method thereof.
2. Description of the Prior Art
An active matrix display uses transistors as switching elements for pixel scanning, of which TFT LCD is a well known example. FIG. 1 is a circuit diagram of a conventional active matrix display. The conventional active matrix display comprises transistors 101 arranged into a matrix, scan lines 102 connecting the gates of the transistors in the same line of the matrix, data lines 103 connecting the sources of transistors in the same row of the matrix, common electrodes 104 corresponding to the transistors 101, capacitances 105 formed between the transistors 101 and corresponding common electrodes 104 and a driver 106.
The driver 106 generates scan signals SS to the gates of the transistors 101 through the scan lines 102 to sequentially turn on or off the transistors 101 line by line. The driver 106 also generates data signals DS to the sources of the transistors 101 through the data lines 103, wherein the capacitance 105 stores one data bit of the data signal DS on the data line 103 when the corresponding transistor 101 is turned on by the scan signal SS on the scan line 102. Thus, the data of the pixels in the matrix is stored and refreshed line by line.
In a conventional active matrix display, Dot Inversion is used to eliminate the Coupling Effect of the capacitances 105 occurring upon the switching of the transistors 101, wherein the polarities of the data signals received by the sources of the adjacent transistors 101 are opposite.
FIG. 2 is a diagram showing the characteristic curve of the data signal used for an 8-bit grayscale image. The data signal DS is a digital signal having digital values 00H˜FFH represented by discrete voltage levels VN1˜VNn and VP1˜VPn with reference to the ground voltage reference VCOM of the corresponding common electrode. Each of the values 00H˜FFH is represented by one of the voltage levels VN1˜VNn when the polarity of the data signal DS is negative, and is represented by one of the voltage levels VP1˜VPn when the polarity of the data signal DS is positive.
FIG. 3 is a circuit diagram of a generator for the voltage levels VN1˜VNn and VP1˜VPn. The generator comprises resistors R0˜RM connected in series. A voltage VDD is applied to the first resistor R0 and the last resistor RM is connected to ground GND. The voltage levels VN1˜VNn and VP1˜VPn are output from the terminals between the resistors R0˜RM.
FIG. 4 schematically shows Dot Inversion applied to an active matrix display. The squares represent where the transistors 101 are, and “+” and “−” represent the positive and negative polarity of the data signal DS received by the transistors 101. In each line of transistors 101, any two of the adjacent transistors 101 receive the data signals DS of opposite polarities.
However, in the previously described conventional active matrix display, the voltage VDD must be twice that of the highest voltage level representing the digital values of data signal DS since the VDD is cut into two halves, one half above the VCOM, for the positive data signal DS and the other half for the negative data signal DS. This increases the cost of the driving IC.
Additionally, the relationship between the voltage levels VN1˜VNn and VP1˜VPn must be VP1>VP2> . . . >VPn>VCOM>VN1>VN2> . . . >VNn for the simplicity of the generator circuit. Thus, the conventional active matrix display is a Normally White system and it is difficult to switch it to a Normally Black system.